Controller Design for Hydraulic Position Control Systems

In recent years, a great deal of interest has been oriented towards hydraulic systems which are energy efficient, responsive (tracking), and accurate. The traditional approach to achieving responsive and accurate positioning performance is to use a servo valve actuator and position and/or velocity feedback. An alternate positioning system is an electrohydraulic actuation system (EHA), in which the fluid from the hydraulic motor is directed back to the inlet of the pump. Changing the swashplate angle or varying the prime mover shaft speed varies the flow to the hydraulic actuator (linear or rotary) which in turn is used to control the positioning or speed of the load. Because there are no major losses associated with throttling of the fluid, power losses are minimized. In earlier EHA systems, the actuator was limited to that of a rotary system because of the requirement for symmetry in the flow to and from the motor. Recent design changes to linear single rod actuators have expanded the EHA applications to linear positioning. In addition, a specially designed EHA linear actuator system was shown to be able to position a load to 200 nanometers. However, the ability to track a desired input path was not extensively studied and as such, algorithms to control this high precision EHA system were required; hence this was the motivation for this study. The main objective of the thesis was to develop high performance control schemes for (1) a valve controlled hydraulic positioning control system (HPCS) and (2) a specific precision positioning EHA system and verify their position tracking performance. Control methods that were applied to HPCSs in the past decade were comprehensively reviewed in this dissertation. Many successful control algorithms have been developed for hydraulic transmission systems, however, certain problems such as slip-stick friction, uncertainty and nonlinearity in hydraulic actuators, pumps and valves are not fully addressed. Three control algorithms are considered in this study: (1) H2-optimal control, (2) H∞ PI plus feedforward control, and (3) robust sliding mode control. The design processes of these three algorithms were based on discrete-time system models. The first two algorithms were based on linear models of the systems while the third applied nonlinear actuator friction in the system model. These three different control algorithms are developed and implemented using simulations and experiments; in addition, their control performance in terms of position tracking and bandwidth performance are examined